1. Field of the Invention
The present invention relates to an overcurrent protection circuit that protects a rechargeable battery from an overcurrent.
2. Description of the Related Art
Conventionally, there is known a protection circuit of a rechargeable battery such as a lithium ion battery or a lithium-polymer battery. FIG. 1 is a circuit diagram of a protection circuit of a rechargeable battery that is generally used. In FIG. 1, a protection package 300 includes a rechargeable battery CELL, connection terminals P+ and P−, and a protection circuit 250. The protection circuit 250 includes an integrated circuit 120 for protecting a rechargeable battery, external resistors R1 and R2, a capacitor C1, a charge control MOS (Metal Oxide Semiconductor) transistor M11, and a discharge control MOS transistor M12.
An N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) having an on-resistance of several tens milliohms is used for each of the charge control MOS transistor M11 and the discharge control MOS transistor M12 in order to monitor charge and discharge currents at a current detection terminal V− by converting the charge and discharge currents into voltages according to the on-resistance and detecting the voltages at the current detection terminal V−. An operation of the charge control MOS transistor M11 is controlled by a voltage at a COT terminal to protect the rechargeable battery CELL by being turned off in an over charge state or an abnormal charger connected state (charge overcurrent state). An operation of the discharge control MOS transistor M12 is controlled by a voltage at a DOUT terminal to protect the rechargeable battery CELL by being turned off in an over discharge state, a discharge overcurrent state or an output short-circuit state. The overcurrent state and the over discharge state can be detected by monitoring a voltage at a VDD terminal.
Here, when a load RL is connected to the battery pack 300 and a discharge current Id flows, a potential Vd at the current detection terminal V− is acquired as Vd=Id×Ron, where Ron is a total value of the on-resistances of the charge control MOS transistor M11 and the discharge control MOS transistor M12. If the discharge current Id increases and the voltage at the current detection terminal V− exceeds a discharge overcurrent detection voltage, the DOUT terminal outputs a low-level signal to turn off the discharge control MOS transistor M12, which results in a discharge overcurrent detection state being set.
At this time, the current detection terminal V− is pulled down to a VSS terminal and pulled in by an overcurrent return resistance RS5 having a resistance of about several tens [kΩ] to several hundreds [kΩ]. Thereby, when the load RL is released, the potential at the current detection terminal V− becomes smaller than discharge overcurrent detection voltage, which permits returning from the overcurrent protection state to a normal state.
In addition, there is suggested a charge and discharge protection circuit, which permits an efficient quick charge (for example, refer to Patent Document 1). The charge and discharge protection circuit includes an overcharge detection circuit, an over discharge detection circuit, a charge overcurrent detection circuit, a discharge overcurrent detection circuit, and a charge control FET and a discharge control FET connected to a charge and discharge circuit in series. The charge and discharge protection circuit protects a rechargeable battery from an overcharge, an over discharge, a charge overcurrent or a discharge overcurrent by turning off the charge control FET when the overcharge detection circuit detects an overcharge and when the charge overcurrent detection circuit detects a charge overcurrent, and by turning off the discharge control FET when the over discharge detection circuit detects an over discharge and when the discharge overcurrent detection circuit detects a discharge overcurrent. The charge and discharge protection circuit forcibly turns on the discharge control FET after a predetermined time has passed if a charger is connected when an over discharge is detected in order to suppress deterioration of the discharge control EFT due to a charge returning from a parastic diode of the discharge control FET.    Patent Document: Japanese Laid-Open Patent Application No. 2007-325434
However, in the conventional technique illustrated in FIG. 1, because the resistance of the load RL is normally much larger than the resistance of the overcurrent return resistor RS5, the potential Vd at the current detection terminal V− is nearly equal to a potential at a connection terminal P+(Vd≈P+(=VDD)). Here, a leakage current flows from the connection terminal P+ to VSS (ground potential) through the above-mentioned overcurrent return resistor RS5. If it is assumed, for example, that VDD=4.0 [V] and RS5=50 [kΩ], the leakage current Ileak is 77 [μA]. Because the current consumption of the rechargeable battery protection integrated circuit 120 is at a level of several microampares [μA], the value of the leakage current Ileak is very large relative to the current consumption of the rechargeable battery protection integrated circuit 120. That is, a large leakage current Ileak flows although the rechargeable battery is protected by the protection circuit 25, which raises a problem in that a service life of the rechargeable battery CELL is reduced.
As measures for preventing such a reduction in the service life of the rechargeable battery CELL, it is considered to reduce the leakage current Ileak by increasing the resistance of the overcurrent return resistance RS5. FIG. 2 is a circuit diagram of a part of the protection circuit 250 of the rechargeable battery CELL including the overcurrent return resistor RS5 and elements associated with the overcurrent return resistor RS5. In FIG. 2, if the resistance of the overcurrent return resistor RS5 is simply increased, the potential Vd at the current detection terminal V− is increased due to a current Iv− flowing into the current detection terminal V−. Thus, the potential Vd at the current detection terminal V− becomes larger than the discharge overcurrent detection voltage, which raises a problem in that it cannot return from the discharge overcurrent protection state. For example, if it is assumed that the current Iv− flowing into the overcurrent return resistor RS5 from the current detection terminal V− is 500 [nA] and the discharge overcurrent detection voltage is 100 [mV], the maximum value of the resistance of the overcurrent return resistor SR5 is 200 [kΩ]. Thus, it is appreciated that there is a restriction in simply increasing the resistance of the overcurrent return resistor RS5.
Additionally, in the structure disclosed in the above-mentioned Patent Document 1, the above-mentioned problem associated with the leakage current Ileak is not considered, and, thus, there is a problem in that a large current flows in the overcurrent protection state.